1. Field of the Invention
The present invention relates to an X-ray computer tomography (CT) apparatus, and particularly, to an X-ray CT apparatus that can prevent displacements of an X-ray source and an X-ray detector that are mounted on a rotation frame provided in a gantry of the X-ray CT apparatus when the rotation frame rotates at high speed.
2. Discussion of the Background
Usually, an X-ray CT apparatus includes, as illustrated in FIG. 8, an X-ray tube 20 and an X-ray detector 30 provided on a vertical circle member of a rotation frame 10 in a gantry so as to face each other through a center aperture 10C of the gantry. An object P is inserted into the center aperture 10C of the gantry. A horizontal drum member of the rotation frame 10 is rotatably supported by a fixing base 100 fixed on a floor. Thus, a plurality of bearings 60 is provided between an outer surface of the drum member of the rotation frame 10 and a horizontal drum member of the fixing base 100. Consequently, X-ray tube 20 and X-ray detector 30 provided on the rotation frame 10 face each other through the aperture 10C are continuously rotated around an objected inserted into the center aperture 10C of the gantry while supported on a top plate 70. X-ray beams irradiated from the X-ray tube 20 through a diaphragm mechanism 90 penetrate the object P and are detected by the X-ray detector 30 as a plurality of X-ray projection data. On the rotation frame 10, other devices, such as a high voltage generation apparatus for supplying a high voltage to the X-ray tube 20 and a cooling apparatus for cooling the apparatus, are also mounted.
FIG. 7A is a simplified illustration of the positional relationship between the rotation frame 10 and the bearings 60 illustrated in FIG. 8. As shown in this figure, a plurality of devices, such as, an X-ray tube 20, an X-ray detector 30, a high voltage generation apparatus 40 and a cooling apparatus 50, are mounted on a vertical circle member of the rotation frame 10 in the gantry of the apparatus. The rotation frame 10 is rotatably supported by a fixing base through a plurality of bearing units that are provided between an inner surface of the drum member of the rotation frame 10 and an outer surface of a drum member of the fixing base.
The rotation frame 10 is usually made of an aluminium (Al) casting. Consequently, when a rotation speed of the rotation frame 10 is increased in order to increase observation efficiencies, distortions of the rotation frame 10 occur due to rotation accelerations that are given in accordance with the weight of the rotation frame 10. For instance, rotation distortions may occur along directions as shown by arrows in FIG. 7B due to high speed rotation. FIG. 7B shows a case where X-ray tube 20 and X-ray detector 30 are slightly displaced so as to shorten a predetermined distance between them due to distortions of the rotation frame 10. Of course, in contrast to the arrows shown in FIG. 7B, it may occur that X-ray tube 20 and X-ray detector 30 are displaced so as to expand the predetermined distance between them as may occur case by case. Such displacements of X-ray tube 20 and X-ray detector 30 due to a high speed rotation of the rotation frame 10 generate errors in the acquired image data through the detector and deteriorate the quality of reproduced images based on incorrect data. It may become a serious problem for an image diagnosis apparatus.
To reduce such distortions due to a high speed rotation of the rotation frame, it has been proposed, for example, as suggested in Japanese Patent Application Publication 8-102550, to provide appropriate weight members on the rotation frame in order to prevent displacements of the rotation frame due to rotation and to maintain balance of the rotation frame. However, such a construction adds weight and makes the apparatus complicated and heavy.
Incidentally, in an actually used X-ray CT imaging apparatus of a high speed rotation, a rotation T for acquiring a necessary projection data of an object by rotating an X-ray source one rotation around an object takes about 0.5 second. Thus, a rotation frame in a high speed imaging X-ray CT apparatus needs to rotate in a high speed of two revolutions during one second.
Generally, supposing that a revolution radius is R (cm), a relationship between the rotation time T and a centrifugal acceleration G is represented by the following equation (1).G=0.0402(R/T2)  (1)
For example, in a case the revolution radius is be 75 cm, the centrifugal acceleration due to the frame rotation becomes about 12 G. Accordingly, it will be understood that the rotation frame having an X-ray source, an X-ray detector and various apparatus in a high speed X-ray CT imaging apparatus receives a large acceleration during rotation. When a rotation radius R of X-ray source is 75 cm, centrifugal acceleration to the rotation frame during a rotation becomes about 12 G from the formula (1). Thus, in a high speed imaging X-ray CT apparatus, the rotation frame made of aluminium and various devices mounted on the rotation frame, i.e., an X-ray tube, an X-ray detector, a high voltage generator and cooling apparatus, also receive a large centrifugal force during the rotation. Nevertheless, to increase observation efficiencies for instance, it is desired to increase one rotation time of X-ray tube and X-ray detector up to 0.25 second, i.e., 4 rotations per one second. In this case, even when a rotation radius R is 75 cm, the centrifugal acceleration during a high speed rotation becomes about 48 G from the formula (1). If a rotation radius also is increased in order to increase observation efficiency, the centrifugal acceleration is further increased.
To prevent such occurrence of distortions of the rotation frame 10 due to a large centrifugal acceleration, it is considered to strengthen a rigidity of the rotation frame 10 itself by making an iron casting and strengthening a rigidity of the bearings by using large size bearing units. However, since rotation inertia of the rotation frame itself also increases in accordance with the increase of the rigidity of the rotation frame during a high speed rotation, it becomes necessary to strengthen a braking force in order to stop the rotating frame. Consequently, an X-ray CT apparatus becomes much larger and heavier as a whole when measures are taken to strengthen the bearing units and the rotation frame.
As explained above, when a rotation speed of a rotation frame in an X-ray CT apparatus is increased, the rotation frame receives a larger centrifugal force. Nevertheless, it is strongly desired to achieve a much higher speed X-ray CT apparatus having a much larger aperture radius of the rotation frame in order to increase observation efficiency of an X-ray CT apparatus.